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DomB

Chaps, day after tomorrow I need to do my first insulation test on a PV array.

It's roof mounted & class II, so the frame is not bonded.

I use a Megger Tester. Am I right in thinking I plug the whole string togethor then put one of the tester's probes into the DC positive plug and the other on the array's roof mounting or the building's earth and then do a 500V insulation test? Then do the same for the negative?

Thanks!

p.s. and to do a Polarity check I guess I just make sure the V is not negative when I put a tester between the DC cables?

p.p.s and to do an earth continuity - I guess that's 'Not Applicable' for a class II system!?

Many thanks!!
 
Last edited by a moderator:
Hi, no answer but then I guess i did hammer out a raft of questions on one thread there!

We do the insulation test (against the mounting system) and DC polarity tests as I mention above. Earth continuity on the DC side hasn't been necessary on any of our installs yet - no lightning protection or transformerless inverters; will need to read up on that soon enough tho!

Cheers
 
Looks like no reply then, We shorted the positive to negative of the array and tested at 250v between that point and the frame, would be nice to see some advice on this test though , as sending 500v through 3 grands worth of panels takes bravery !! and no matter how much I search on line I cant find much in the way of advice
 
Without the string cable connected to any of the modules connect both ends together on the roof so essentially you would have one long cable. On one of the open ends of the cable (positive or negative it diesnt matter) inside the house measure the insulation resistance to the main main earth on the ac side. Hope this makes sense
 
Yes, makes sense this would test the integrity of the feed cables from inverter to array +ve and -ve. Doesnt however check the cables between panels or the panels themselves. (But these would have been tested when manufactured I suppose they could have been traped against frame etc or damaged in transit) Thanks for your reply, shame there isnt a bit more info on this out there. Trained sparky or not best practice is nice if its availlable for all to read.
 
I doubt they would get damaged in transit they are usually taped to the tedlar witch will be about 50mm away from the module underneath it but I guess it could be possible but then again any damage from being trapped would be visible
 
Just on that point of putting 500v through the array, I figured a string of enough panels, say 15, would probably generate around that much themselves. And the tester must be using DC. Maybe 15 is a very long string but I figured it wasn't totally outrageous.
 
Another point to add is that the inverter itself is also an insulation resistance tester, some inverters will tell you the value and others wont, but i believe all of them will shut down if the insulation resistance falls to below 1 mega ohm and log it as a fault
 
Yes the sunnyboy we put in does shut down if the array has a ground fault. Its just that the comisioning document asks for an insulation test and as we were new to the game so were trying to be really concensious. Yet in the training and in all we could find on line there was not much said. Strange we also havenr seen other installers doing to much in the way of testing in this area, but a bit of a debate is always good to see what people think. Thank you for your replies
 


Dti pv in buildings guide:
Two test methods are possible:
a) Test between Array Negative and Earth followed by a test between Array Positive and Earth
b) Test between Earth and short-circuited Array Positive & Negative​
Where the structure/frame is bonded to earth, the earth connection may be to any suitable
earth connection or to the array frame (where the array frame is utilised, ensure a good
contact and that there is continuity over the whole metallic frame).

For systems where the array frame is not bonded to earth (eg where there is a class II
installation) a commissioning engineer may choose to do two tests: i) between Array cables
and Earth and an additional test ii) between Array cables and Frame.

For Arrays that have no accessible conductive parts (eg PV roof tiles) the test should be between

Array cables and Building Earth

quite straight forward ,but note test between array and building earth only necessary for pv tiles ,anything else ie retro or inroof will have accessible conductive parts
 
Ok this is in line then with what we did. First mesured + and - to earth in voltage terms to check no obvious ground fault. Result as expected slight kick on connection but zero volts then shorted + to - in loft. (loop current now running as pannels are connected) connect wander lead to frame first. (reducing risk of shock). Then insulation test at 250v from frame to point of + and - short. Should have probably been at 500v ie twice normal voltage (Which for this array was 208v) but I didnt fancy 500. I was sort of hoping that someone would say fine mate we do it like this etc........ Thanks for your post
 
Check the module data sheet most will say max system voltage 1000v so 500 is'nt a problem
when I did pv niceic course the electrician who was supposedly showing us what was what , did'nt know how to test the arrays
so we got him sweating, when we started testing his nice shiny new modules 1st at 500 then at 1000v but there was no effect on the output whatsoever. but again it pays to check first
 
I know one MCS accreditor that ignores this test because they cannot relevance to a classII install?!! Regs show 2 methods, and we test between combined live conductors to frame. You need to have a short circuit box~ I'm told Schuco make one but have never tracked one down. You can make one out of a DC isolator and even add in std meter leads with 4mm plugs. You can use the DC isolator by shorting out the inverter connections....
There are test voltages given in BS EN 62446:
taking I as I oc stc x 1.25 then if I= below 250 test at 250;120 to 500 test at 500V,above 500 test at 1000V;but you can reduce the test voltage if it would be higher than the max module voltage (see module data sheet). Most modules have a max working voltage of 1000, but thats your first check.
I believe that the 17th edition is going to have a large amendment this year (March was the goal) and 712 is going to be major, probably a full adaptation of 62446 which BS has already adopted. I've never installed a grounded system, but imagine that could be interesting. Hope this helps
 
Yes, I feel im geting some understanding of this now, bit confused by this bit though " if I= below 250 test at 250;120 to 500 test at 500" Did you mean 120 ?
So if the open circuit array voltage is 208v what is the test voltage. Have you ever seen people do this test ????

Dont have a copy of BS EN 62446 cant find it free online. lots of dosh.... but perhaps my friend (MCS next week hopefully does !!) Im just a domestic Part P man with a background in electronics and UPS systems.


We did the test by using the isolator and 2 bits of cable in a test block not scientific but in fault condition situation with non shaded panels we could have had 208v on the test leads. Not something you should climb a wet ladder with etc so a carefull approach is a good idea. We clipped lead to frame first and tested from loft. Would be nice if this test was really spelt out in english why does this stuff all have to be so hidden, is it because people, dont know, cant make there mind up, or all know but keep it secret.

In searching for this ive now discovered a whole no game of too earth frames or not that seems to be another black art.

Many thanks
 
Sorry yes 120 should be 250.

BS 62446 is a bit of a bible for PV. I've heard that Liverpool main library have all BS copies on line for free if your a member! (costs to become a member)I think our GB regulators have been spending to long getting on top of this, after all, were importing IEE regs in PV that have been refined over years.

Black arts~ bulls**t. The guys you presume are in the know may not have a clue. I asked a new PV training provider about the testing measures and he stumbled! Bums on seats scenario.

The concerns in testing are short-circuiting the DC and arcing, along with high DC voltages that might go with the system., and then the insulation resistance charge~ hanging off the edge of a roof is not ideal especially at 1000v. Neither is curious clients and their kids. Testing recommends live conductors (combined) to each part of the frame~ ie 2 rows of modules = 4 roof bars = 4 tests.
Individual live conductors to earth is a curious one. If not classII then a potential may be picked up and the meter cut out as it's meant to if over 50v. Spoke to the guys at Megger last year over this, and they couldn't see how it could be achieved. Test with AC voltage? Be interesting to hear from anyone who knows the europeans method of testing.
 
Hampshire library membership also gives full online access to the BS website (not for commercial use, reference only) and is free to join if you are lucky enough to live in Hampshire. I suspect all UK libraries have a similar arrangement.
 
"Individual live conductors to earth is a curious one. If not classII then a potential may be picked up and the meter cut out as it's meant to if over 50v."

I'm not sure what you mean ?

if your doing insulation resistance test between + or - (the same conductor) then you would have the two ends of the string disconnected
so that no circuit is there ,no circuit no voltage , test between + to earth and - to earth .
 
Thats how the IEE regs state it as a second test method (with open circuit the GS38 meter detects the Voc and cuts out). Maybe it applies to main DC cable runs only ie with strings isolated as in our standard insulation tests. Or test applies for middle grounded systems (3wire systems). Theory may be that test current can't flow through modules and their cells anyway as not part of semi-conductor flow. I haven't been able to get any info. on this testing method, does anyone know what it applies to?
 
Sorry yes 120 should be 250.

BS 62446 is a bit of a bible for PV. I've heard that Liverpool main library have all BS copies on line for free if your a member! (costs to become a member)I think our GB regulators have been spending to long getting on top of this, after all, were importing IEE regs in PV that have been refined over years.
This sounds like something we could do with - has anyone actually got a copy - is it worth it?
 

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